Hemodynamic monitoring allows the clinician to have access to information that is not available from a standard assessment of the cardiovascular system. Parameters such as cardiac output (CO) and intracardiac pressures can be directly measured and monitored through an indwelling catheter connected to pressure monitoring equipment. Intracardiac pressures include, among others, pulmonary artery (PA) pressure, radial artery (RA) pressure, and pulmonary capillary wedge pressure (PCWP).
Hemodynamic monitoring systems include one or more indwelling catheters, each connected to a pressure transducer, a flush system, and a bedside monitor. For example, the PA catheter is a multilumen catheter inserted into the pulmonary artery. The arterial catheter, or “A-line,” has only one lumen which is used to directly measure arterial blood pressure. Transducers are used to sense vascular pressure in the catheters. A pressure transducer is a small electronic sensor which converts a mechanical pressure (i.e., vascular pressure) into an electrical signal which is displayed on the pressure amplifier or bedside monitor as a continuous waveform with corresponding numerical displays of measurement.
The transducer is typically housed in a plastic connector that also includes a port for connection to an IV solution which is placed in a pressure bag. This allows a slow, continuous infusion of fluid through the vascular catheter. For the transducer to work accurately, the transducer generally must be leveled to the catheter tip. Leveling is the process of aligning the tip of the vascular catheter horizontal to a zero reference position using a stopcock in the pressure tubing close to the transducer. The leveling location is the phlebostatic axis which is located horizontal to the 4th intercostal space at the midaxillary line. This coincides most accurately with the atria of the heart.
There are two basic methods for leveling. When the transducer and stopcocks are mounted on a pole close to the bed, the pole height is adjusted to have the stopcock opening level with the catheter tip. To ensure horizontal positioning, a carpenter's level is typically used. The transducer is then “zeroed” to compensate for any distortion by using the stopcock to expose the transducer to air and setting a zero button on the bedside monitor. Each time the bed height or patient position is altered, this leveling procedure is repeated.
Another method for leveling places the transducer and stopcock at the desired location on the chest wall or arm. Ordinary medical tape is used to strap the transducer to the appropriate location on the body to help eliminate the need for repeating the leveling procedure when bed heights are changed. Medical tape, however, can be uncomfortable for the patient, awkward for the caregiver to use, difficult to remove and reapply, can tend to become undone, and increases the likelihood that the caregiver will inaccurately locate the transducer, leading to inaccurate pressure readings.